Energy & Fuels 1991,5,477-482
477
Infrared and Nuclear Magnetic Resonance Spectroscopy of Density Fractions from Callide Coal A. M. Vassallo,* N. C. Lockhart, J. V. Hanna, and R. Chamberlain CSIRO Division of Coal and Energy Technology, P.O. Box 136,N.Ryde, NSW 2113,Australia P. C. Painter and M. Sobkowiak Polymer Science Program, Pennsylvania State University, State College, Pennsylvania 16802 Received September 24,1990. Revised Manuscript Received February 22, 1991 Density-separated fractions of a demineralized Australian subbituminous coal (Callide) were prepared and analyzed by infrared and solid-state I3C nuclear magnetic resonance spectroscopies. The carbon aromaticity V), of the density fractions varies from 0.37 for the fraction which floats at 1.20 g cm-3 Infrared . spectroscopy was used gravity to 0.92 for the sample collected at density 1.44-1.45 g ~ m - ~ with band deconvolution to estimate the change in aromatic C-H, and aliphatic CH3, CH2, and CH absorptions with increasing density. The absorption due to aromatic C-H stretching vibrations is very weak in the lightest density fractions but increases rapidly beyond a density of 1.40 g cm-S and comprises approximately 50% of the total infrared C-H stretching absorption at a density of 1.445 g~m-~ Methyl . absorptions remain relatively constant until density 1.425 g cmJ after which they increase significantly. Methylene absorptions decrease steadily from the fraction with lightest density until a density of 1.40 g cm-3 after which they decrease at a faster rate. Absorptions due to methine groups appear to increase with density up to 1.275 g and then decrease at higher densities. These trends indicate a decreasing component of CH2 and an increasing CH3 and aromatic C-H component within the coal structure as the density of the coal increases. Accelerated changes are evident at and beyond a density of -1.40 g ~ m - ~These . results demonstrate the highly variable chemical structure found within different density fractions of one coal sample.
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Introduction Considerable efforts to separate coal into its physical constituents, i.e., different macerals and mineral components, have been undertaken in recent years in order to investigate its heterogeneity. Macerals can be broadly classified into three groups:' (i) vitrinite, (ii) liptinite (sometimes called exinite), and (iii) inertinite. Vitrinite is the dominant marceral group in coal, especially in Carboniferous Northern Hemisphere coals, and is believed to have arisen from the lignin components of trees, branches, and roots. Some vitrinites may also incorporate submicroscopic algal material. Vitrinite can be further subdivided, based on its microscopic appearance. Liptinite macerals derive from the resinous and waxy parts of plants, such as spores, resin, and cuticles, and usually constitute less than 20% by volume of any particular coal, although they may occur in some instances in large lumps or bands. Inertinite macerals, especially those in Carboniferous coals, are mostly derived from degraded woody tissue which has been carbonized or suffered fungal attack,' although some degraded resinous materials have also been classified as inertinite. However, it has recently been proposed that much of the inertinite in Permian coals of Australia has formed from a type of "freeze drying" in which gellified humic material has been dried without significant oxidation.2 Inertinite is so named because of its supposed inertness during carbonization and liquefaction, but studies existw which show that in some instances ~
~~
(1) Stach, E. In Stach's Textbook of Coal Petrology, Stach, E., Mackomky, M-Th., Teichmuller, M., Taylor, G.H., and Chandra, D., Eds., Gebruder Borntrager: Berlin, 1975. (2) Taylor, G. H.; Liu, S. Y.; Diessel, C. F. K. Int. J. Coal Geol. 1989, 1-22. (3) Mitchell, G. D.; Davis, A.; Spackman, W. In Liquid Fuels from Coal; Ellington,R. T., Ed.;Academic Press; New York, 1977; pp 255-270. (4) Given, P. H.; Spackman, W.; Davis, A.; Walker, P. L.; Lovell, H. L.; Coleman, M.; Painter, P. C. Quarterly Technical Progress Report for Period Jan-June 1978; U.S.DOE Report BE-2494-718.
0887-0624/91/2505-0477$02.50/0
it behaves similarly to vitrinite, particularly in many Permian Gondwana coals. Subdivisions in the inertinite classification also exist and are based mainly on size, reflectance, and morphology. For example, fusinite is high in reflectance and may show cellular structure. It is generally the densest maceral and has a lower H/C ratio than any other maceral. Relatively pure maceral concentrates can be prepared by combining the techniques of hand-picking, chemical demineralization, and density fractionation. The density of coal macerals increases in the order liptinite < vitrinite < inertinhe for any particular coal up to a rank of about 90% C, at which the densities of liptinite and vitrinite tend to similar values! It follows that maceral concentrates can be prepared by density separation techniques, the first detailed study being carried out by van Krevelen et al.s Significant early investigations of hand-picked maceral concentrates were made by van Krevelen and co-workers.&' The British coal sampless have been reanalyzed concurrently with the recent program of characterization of U.S coals at Pennsylvania State University!Jo More recently, the technique of density gradient separation (DGS) has been applied to c081911-13and has allowed very fine density resolution of macerals, albeit on a small scale. (6)(a) Dieseel, C. F. K. Fuel 1983,62,883-892. (b)Dieseal, C. F. K. R o c . 2nd Aust. Coal Sci. Conf. 1986, 327-332. (6) Dormans. H. N. M.. Hunkens. - . F. J.: van Krevelen. D. W. Fuel 1957, 36, 321-339. (7) Van Krevelen. D. W.: Schuver. - . J. Coal Science: Eleevier: Amsterdam, 1957; p 240. (8)Fenton, G . W.; Smith, A. H. V. Gas World 1989, 149. (9) Given, P. H.; Spackmann, W.; Davis, A,; Zoeller, J.; Jenkins, R. G.; Kahn, R. Fuel 1984,63, 1655-1659. (10) Davis, A.; Kuehn, D. W.; Painter, P. C. US.Department of Energy, Final Report, Part 4. DOE PC/30013-4. 1983. (11) Dyrkacz, G . R.; Horwitz, P. hcel 1982, 61, 3-12. (12) Dyrkacz, G. R.; Bloomquiet, C. A. A,; Solomon, P. R. Fuel 1984, 63, 536-542. (13) Dyrkacz, G . R.; Bloomquiet, C. A. A.; Ruacic, R. Fuel 1984,63, 536-542.
1.
0 1991 American Chemical Society
Vassallo et al.
478 Energy & Fuels, Vol. 5, No. 3, 1991
Table I. Microlithotypeaand Proximate Analysisbof Bore Cores from Callide Coal vitrinite clarite durite fusite intermediates shaly coal clay moisture ash volatile matter fixed carbon
A 9 0 70 7 10 3
B 23 2
43 11
10
1
7 4
13.3 8.3 22.5 55.9
10.8 14.6 26.6 48.0
"Vol % basis as received. * Wt % basis.
Spectroscopic analysis of density-separated maceral concentrates has been carried out on relatively few C O ~ ~ S especially , ~ ~ - ~ ~the Permian or Triassic coals from Australia.lg These studies have shown the large variation in structural characteristics that can be found in any one particular coal, such as carbon aromaticity14l&l8functional group distribution and pyrolysis p r o d u ~ t s . ' ~ Pandolfo J~ et al.19 examined high-purity maceral concentrates from an Australian Bituminous coal (Blair Athol) using FTIR and GC/MS techniques, but to our knowledge there have been no other studies of density fractions from Australian coals. In this work, we use density separation techniques to fractionate an Australian coal (Callide) and apply FI'IR and solid-state NMR techniques to their analysis. In particular we attempt to resolve the aliphatic and aromatic C-H infrared stretching absorptions and relate the component absorptions to chemical structural changes as a function of density.
-
200
0
100
Chemical shift. &ppmJ
Figure 1. Solid-state 13C spectra of (a) Callide coal and (b) demineralized Callide coal.
Experimental Section Callide coal is classified as subbituminous class A and has a high proportion of the inertinite maceral semi-fusinite (>SIvol %). Two bore cores were selected from a library in an attempt to obtain reasonable concentrates of liptinite, vitrinite, and inertinite macerals from the same Callide coal deposit. The microlithotype and proximate analysea of these bore corea, deaignated A and B, are given in Table I. All but two density fractions were obtained from bore core B. The two highest density fractions (1.44-1.45g cmJ and 1.45 g cm-s sinke) were obtained from bore core A. These bore cores were subsampled, selectively crushedm to a nominal 1 mm top size, demineralized by using sodium hydroxide solution,2l and sized at 250,63, and 25 pm. Element analyses of the coal before and after demineralization and petrographic composition of the starting coal are given in Table 11. Selected size fractions were then density separated, using low boiling inert liquids and static float-sink procedures.= These techniquesm@ (14) Zilm, K. W.; Pugmire, R. J.; Lartar, S. R.; Allan, J.; Grant, D. W. Fuel 1981,60, 717-722. (15) Pugmire, R. J.; Zilm, K.W.; Woolfenden, W.R.; Grant, D. W.; Dyrkacz, G.R.; Bloomquist, C. A. A.; Horwitz, E. P. Org. Geochem. 1982,
4.7%&4. -,.-
(16) Karas, J.; Pugmire, R. J.; Woolfenden, W. R.; Grant, D. W.; Blair, S. Int. J. Coal Geol. 1985,5,315-338. (17) Choi, C.; Dyrkacz, G . R.; Stock, L. M. Energy Fuels 1987, I, 2Rn-2%. -- - ---.
(18) Choi, C.; Wang, S.; Stock, L. M. Energy Fuels 1988, 2, 37-48. (19) Pandolfo, A. G.; Johns, R. B.;Dyrkacz, G.R.; Buchanan, A. S.
Energy Fuels 1988,2,657-662. (20) Lockhart,N. C.; Chamberlain, R. "Selective Breakage of Coal and Beneficiation by Dry Methods". NERDDP Report EG89/792,1988, Vol.
2, 110 pp. (21) Waugh, A. B.; McG. Bowling, K.Fuel Process. Technol. 1984,9, 217-233. (22) Lockhart, N. C.; Hart,G. H.; Smyth, M. In Workshop on Steaming Coal: Teeting and Characterbation: Gupta, R. P., Wall, T. F.,%.; Institute of Coal Reeearch, Newcastle University Newcastle, NSW, 1988; pp N27-N29.
4000
3500
3000
2500
2000
1500
io00
5dO
UaVonUmbor8
Figure 2. FTIR spectra of (a) Callide coal and (b) demineralized Callide coal. were devised to ensure reliable separations of large samples down to 5 pm top size while preserving optically recognizable macerals. Selected size/density fractions were then subjected to analysis as per Table m. It should be noted that aqueous or organic liquid density measurements may overestimatethe true helium density for low rank coals2)such as Callide. With'this in mind, referencea to the density of coal fractions herein do not necessarily equate with the true density. The density of each fraction was taken as the mid-value of the density range. Petrographic and carbon and hydrogen analysea were carried out on those fractions of which there was sufficient sample. The resultsof these analysea are given in Table IV. The FTIR spectra were obtained on a Digilab FTS-60 spectrometer using KBr pellets (200 f 1mg)containing -0.5% w/w of the coal sample. Between 64 and 256 scans,at nominal resolution of 2 cm-', were obtained prior to Fourier transformation. Band resolving was carried out on the 2700-3700-cm-' region of the spectrum. Absorptions due to 0-H groups were fitted to four (23) Franklin, R. Fuel 1948,27, 46-49. (24) Bellamy, L. J. The Infrared Spectra of Complex Molecules; Chapman and Hall: London 1975; Vol 1.
ZR and NMR Spectra of Callide Coal Fractions
Energy & Fuels, Vol. 5, No. 3, 1991 479
Table 11. Element Analyses and Petrographic Composition of Callide Coal % Ca % Ha % N" % Sa % 0" % moist. % ash 96 vit callide coal 76.0 4.13 1.7 0.2 18.0 9.25 14.54 43.0 demineralized callide coal 78.2 4.12 1.6 0.2 15.9 4.01 0.74
% liD
% inert.
7.5
49.5
" Daf basis, 0 by difference and rounded to one decimal place. Table 111. Mass Yields of Selected Size/Density Fractions of Callide Bore Cores size fraction bore core A re1 density, bore core B -63 + 25 -63 + 25 code g c m 4 - . -250+63rm pm rm CALl C1.20 8.7" 1.4 CAL2 1.20-1.25 5.6' 3.1a 1.4 CAL3 1.25-1.30 CALI 1.30-1.35 40.4" CAL5,6b 1.35-1.40 39.1a 74.6' 3.6 CAL7 1.40-1.41 3.1 12.7' 2.3 CAL8 1.41-1.44 9.S 3.6 CALS 1.44-1.45 79.9 CALlO >1.45 10.6'
Table V. Typical Parameters for Curve Resolving of the C-H IR Stretching Region band width at wition, cm-* originU half-height. cm-* 3050 f 10 aromatic C-H 56 2952 f 4 CH3 32 2924 f 3 CH, and CH3 40 2897 f 4 CH 27 2883 f 7 CH, 27 2851 f 4 CH2 34
'Sample taken for analysis. bCAL5corresponds to the -250 + 63 pm fraction while CAL6 corresponds to the -63 + 25 pm fraction. Table I V Petrographic Comporition of Density-Separated Fractionr from Callide Coal demity fraction, I % I gcm-S code ICa % Ha liptinita vitrinita inertinite C1.20 CALl 74.7 7.67 77.3 17.5 5.2 1.20-1.25 CAL2 N/D N/D 78.5 18.1 3.4 1.25-1.30 CAL3 73.2 7.17 13.7 34.0 52.3 1.30-1.35 CAL4 72.0 6.04 41.6 22.2 36.2 1.35-1.40 CAL5 73.8 5.60 18.9 54.3 26.8 1.40-1.41 CAL7 N/D N/D N/D N/D N/D 1.41-1.44 CAL8 70.0 3.95 1.0 4.3 94.7 1.44-1.45 CALS 70.2 4.12 1.5 97.7 0.8 >1.45 CALlO 72.2 3.99 1.6 0.8 97.6 'Dry basis, N/D not determined. bands, typically at 3470,3215,2950, and 2500 cm-', with bandwidths at half-height of 260,360,520, and 575 cm-',respectively. A least-squares technique was used to obtain the best fit and a synthetic OH absorption band was then subtracted from the sample spectrum. In some cases the baseline was then corrected by using single or multipoint techniques. The resulting region was then resolved into six bands by using the same least-squares procedure. Typical parameters for the curve resolving process are given in Table V. The area of each band was calculated and ratioed to the area of the whole region. Solid-state NMR spectra were obtained on a Bruker CXP-100 spectrometer using magicangle spinning, cross polarization, and high-power proton decoupling. Samples were spun at -3200 Hz and between 10 and 50OOO scans obtained. A recycle delay of 1 s and contact time of 1 ma were used. Chemical shifts are referenced to tetramethylsilane (TMS).
Results and Discussion Demineralization. The effects of caustic demineralization on the organic matter in Callide coal appear to be minimal. The change in elemental composition (Table 11) is very slight, and the solid-state 13C NMR spectra of the coal before and after demineralization (Figure 1) are virtually identical. The FTIR spectra of the coal before and after demineralization are also very similar (Figure 2), except for the loss of the mineral absorptions at -3650, 1000-1100,910, and cm-'. Density Separation. The densities of coal macerals generally fall within the following ranges: (i) liptinite 1.15-1.30 g ~ m - (ii) ~ , vitrinite 1.27-1.38 g ~ m - and ~ , (iii)
< t 2 0 gcm-3
1.44-1.45 gcm-3
200 100 0 Chnnkll shift, blppml
Figure 3. Solid-state 13C spectra of selected density-separated fractions of Callide coal.
inertinite 1.35-1.70 g ~ m -for ~ ,carbon contents less than -92%. Liptinite and vitrinite approach the same density above -90% C and these move toward the same density as fusinite when their carbon value reaches -94%.6 The petrographic data in Table IV reveal how the maceral composition of the fractions changes with density. The two lightest fractions are predominantly (>75%) liptinite. Although a quantitative measurement of the submaceral type was not attempted, it is clear that the majority of the liptinite consisted of cutinite. The mid-range fractions, 1.25-1.40 g ~ m -consist ~ , of mixtures of liptinite, vitrinite, and inertinite with the liptinite and vitrinite content decreasing with increasing density. Based on earlier studies16J6 it would appear that the predominant inertinite maceral in the 1.30-1.40 g cm-3 density range is semi-fusinite while the heavier fractions should be predominantly fusinite. Those fractions with density above 1.41 g cmJ are almost totally inertinite. NMR Spectra. The solid-state 13C NMR spectra of three of the density fractions (C1.20, 1.35-1.40, and 1.44-1.45 g ~ m - are ~ )shown in Figure 3. The resonances at -30 ppm arise from aliphatic carbons, those a t 130 ppm from aromatic carbons,and those at 150 ppm from
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480 Energy & Fuels, Vol. 5, No. 3, 1991
Vassallo et al.
Denrlty (gcm-3)
Figure 4. Change in carbon aromaticity with density for Callide
coal.
aromatic carbons bearing an oxygen substituent, and the resonance a t -177 ppm is due to carboxyl groups. The carbon aromaticity of these fractions, shown in Figure 4, clearly demonstrates a pronounced and almost linear change in aromaticity with density for this coal. The lighest material, predominantly liptinite, has an aromaticity slightly lower than that reported for sporinites"-16 but higher than that reported for alginites.14J6 The band at 177 ppm, arising from carboxyl groups, can be identified with the 1703-cm-' band in the IR spectrum and thus must be due to carboxylic acid groups. The sharp band at 31 ppm is consistent with long-chain polymethylene carbon. The petrographic identification of cutinite in this fraction is consistent with these spectroscopicobservations. Indeed there is a strong similarity in the spectrum of this fraction to the spectrum of Indiana "paper" coal componentsz6 which are known to be mainly cutinite. The fraction with density 1.35-1.40 g cm-3 has a much higher aromaticity (0.74) than the light fraction, as expected, and is within the range found for subbituminous coals. This fraction consista of a mixture of vitrinite, inertinite, and liptinite macerals (Table 111). The heaviest fraction analyzed by solid-state NMR, with a density range of 1.44-1.45 g ~ m -is~highly , aromatic (fa = 0.92) and has a spectrum similar to fusinite.16 The very low aliphatic carbon content is consistent with the very weak aliphatic C-H absorption in the IR spectrum. This fraction is almost entirely inertinite (>97%, Table 111). There have been only a few reports on the solid-state '9c NMR spectra of density separated coal fractions in the 1iterature."-l6J8 Karas et al.18 measured the aromaticity of density fractions of eight coals and obtained a linear plot against density. Stock et al.18 reported the f a values of density gradient separated macerals from West Virginia Upper Kittaning Seam coal and these values also plot close to the line determined by Karas.16 The results reported here are similar to these, although the aromaticities of the fractions from Callide coal are always lower than those of fractions of similar density reported by these workers. More coals need to be studied before any conclusions can be drawn about the possible significance of these lower values. Infrared Spectra. The IR spectra of the material concentrated within each density range are shown in Figure 5. The origins of all the major bands in the IR spectrum of coal have been assigned and a review is
available.% Looking first at the region due to aliphatic and aromatic C-H bond stretching (2800-3200 cm-l), it is evident that the low-density fractions display much more signal intensity in this region than do the high-density fractions. This is consistent with the known changes in hydrogen content and density of coal macerals.8 The signal intensity of this region decreases sharply at density fractions greater than 1.40 g cm-3 and is almost indistinguishable from the 0-H absorptions of the two heaviest density fractions. Absorptions due to aromatic protons (3000-3100 cm-') can be detected in the spectra of all fractions (at high sensitivity) but do not become relatively significant until the 1.30-1.35 g cm-3 density fraction. Bearing in mind the lower absorptivity of aromatic C-H vibrations compared with aliphatic C-H vibrations (from 0.3 to LOz4),it is evident that for those fractions with density greater than -1.41 g cm-3 the ratio of aromatic to aliphatic protons exceeds 1. A more detailed analysis of the C-H stretching region is given below.
(26) Anderson, K. B. The Chemical Characterisation of Macer& bolatad from Australian C d and Sediments. Ph,D Thesis, University of Melbourne, 1989; p 184.
(26) Painter, P. C.; Starainic, M.; Coleman, M. M. Fourier Trqnsform Infrared Spectrometry;Academic P-: New York, 198s; Chapter 6. (27) Brown, J. K.J . Chem. SOC.1966, 744-752.
v,)
1
1
4000
3000
I
I
I
1000 4 5 0 Wavenumber (cm-l)
2000
Figure 5. Infrared spectra of density-separated fractions of Callide coal: (a) CAL1, (h)CALP, (c) CAL3,(d) CALI, (e) CAL5, (0 CAL7, (9) CALS, (h) CALS, (i) CALlO.
IR and NMR Spectra of Callide Coal Fractions In the C - 0 region (1650-1800 cm-') the spectra of all fractions have a band at 1704 cm-', with the lightest fraction showing the strongest intensity, although this may be due, in part, to the much lower intensity of the adjacent 1600-cm-I band in this particular fraction. Absorptions at -1700 cm-' have been observed in many studies of liptinite concentrate^'^*^^ and have been assigned to carboxyl groups in esters and acids, which are remnants of the original plant input to these types of macerals. The presence of this band in the spectra of heavier fractions is undoubtedly due in part to some contamination of these fractions with liptinite (Table 111) or may indicate some degree of oxidation; however, the constancy of the band's position would mitigate against a different origin to that in the lightest fraction. The 1600-cm-' band, arising from aromatic rings enhanced by polar functional groups,n is the dominant band in the spectrum of all fractions except the lightest, but this band is difficult to analyze diagnostically because of its variability in intensity. The band centered at 1450 cm-' arises from the bending mode of aliphatic C-H bonds. The position of this band changes from 1454 cm-l in the spectrum of the lightest fraction to 1433 cm-' in the spectrum of the deneest fraction with proportional changes in the intermediate densities. This indicates a transition from CH2to CH3 structures with increasing density. Absorptions due to CHSgroups are visible in the spectra of all fractions at -1370 cm-'. The region from -1300 to 1100 cm-' is difficult to interpret due to the complexity of the absorptions that can occur in this region, but the spectrum of all fractions display a strong absorption centered at 1250 cm-', probably due to C-O bonds, with 0-H and C-C bonds also contributing. The region between 700 and 900 cm-' is predominantly due to out-of-plane aromatic C-H deformations but may include a component of aliphatic C-H if large amounts are present, particularly long-chain material.31 Absorptions in this region can be used to establish the pattern of aromatic substitution in many cases.% Isolated protons, for example, generally absorb in the range 860-900 cm-'while two adjacent protons generally absorb between 800 and 860 cm-', although the polarity of adjacent substituents may influence their position. In coals,three bands are very often seen in this region (neglecting mineral and aliphatic C-H absorptions). For the density-separated fractions of Callide coal, these bands are at 880,820, and 750 cm-' in all fractions; Le., there is almost no change in the pattern of aromatic ring substitution with increasing density. The two lightest fractions have additional absorbances in this spectral region, at 803 and 720 cm-', due to aliphatic C-H bond deformations. Interestingly, the intensity of the 750-cm-' band increases relative to the 820- and 880-cm-' bands as the density of the material increases. This indicates that the degree of aromatic substitution is less for denser fractions; i.e., the amount of four or five adjacent ring hydrogens is relatively greater in the denser materials. This is consistent with Brown's observationn that the degree of aromatic substitution decreases with increasing rank, up to a value of 94% C. Similar observations have been made using NMR.32*s3 Band Analysis of C-H Infrared Stretching Ab-
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(28) Bent, R.; Brown, J. K. Fuel 1961,40,47-56. (29) Murchiem, D.A.C.S. Symp. Ser. 1966,55,307-331. (30)Millais, R.; Murchison, D.G. Fuel 1969,48,247-268. (31) Kuehn, D.H.; Snyder, R. W.; Davis,A.; Painter, P. C. Fuel 1982, 61,682494. (32) Retcofaky, H. L. Appl. Spectrosc. 1977,31(2), 116-121. (33) Wileon, M. A.; Alemany, L. B.; Woolfenden, R. J.; Given, P. H.; Grant, D.M.; Karaa, J. A d . Chem. 1984,56,933-943.
Energy & Fuels, Vol. 5, No. 3, 1991 481
3000 Wavenumbw (cm-1)
4000
2000
Figure 6. Example of band deconvolution for sample CAL7, ~ 2 0 0 0 - c mregion: ~ 1 (a) real spectrum, (b) syntheticspectrum, (c) component bands.
(a) Aromatic
C-H
( 3 0 3 0 cm-1)
( c ) Methine ( 2 8 9 4 cm-1)
(b) Methyl (2957 cm-1)
(d) Methylene I2923 cm-1)
Figure 7. Relative band areas from deconvolution of the 28003100-cm-' region of IR spectra of density fractions of Callide coal.
sorptions. Least-squares curve fitting of IR absorption bands has been demonstrated to be a useful technique for extracting structural information from coal spectra.2e Although the technique can still be considered to be more an "art" than science, especially for complex spectra, valuable data can nevertheless be obtained by using logical procedures. Included in these procedures is an identification of known and expected bands and the use of rational bandwidths and baseline determinations. The nature and position of the C-H stretching absorption in coals are well-known and documented.% Table V lists the band positions and half-widths used in this work. Although it is well-known that more than one aromatic C-H stretching absorption is present in the spectra of coals,28for the purposes of this study this resolution was not required and this region was adequately fitted with one band. An example of the deconvolution of the C-H stretching region is given in Figure 6 for the 1.40-1.41 g cm4 density fraction to Callide coal. The component band areas of C-H stretching vibrations can be expressed as a percentage of the total C-H stretching absorption area, in order to provide an indication of how the intensity of these bands
Energy & Fuels 1991,5,482-487
482
changes with sample density. These data are shown in Figure 7. The relative increase in aromatic C-H absorption with density is shown in Figure 7a. Bearing in mind the somewhat lower absorptivity of aromatic C-H stretching vibrations compared to aliphatic C-H stretching vibrations,n it is clear that there is a very rapid change in proton distribution within the density range 1.40-1.45 g ~ m (CAL7-CALS). - ~ This change in Ypseudonproton aromaticity is much more rapid than the change in carbon aromaticity, but the former measurement is not quantitative because of the different value of the extinction coefficient for aromatic and aliphatic C-H absorptions. Nevertheless, it does indicate that the proton distribution is changing more rapidly than the carbon distribution with increasing density. Similarly, absorptions due to methyl groups (Figure 7b) appear relatively constant until density 1.40 g cmJ and then increase rapidly above this density. Methine absorptions (Figure 7c) are generally weak and thus measurement of their contribution is subject to larger errors; however, except for sample CAL8, it appears that the methine absorption increases with density up to a value of 1.27 g cmd then drops sharply for higher densities. Methylene absorptions (Figure 7d) decrease with increasing density until they comprise only -15% of the total C-H signal at density 1.445 g ~ m - ~These . results are in agreement with the aliphatic group distribution obtained by oxidation techniques on a slightly higher rank (carboniferous) coal by Stock et al.18 Taking into account the possible variation in absorptivity between the different aliphatic structures, it is believed that the results presented here demonstrate, at least semiquantitatively, the expected range of, and variation in, C-H distribution within density fractions of one coal.
Conclusions
The density separation of coal enables a more detailed spectroscopic study of the structural variability found within a particular coal. For Callide coal, density separation and ancillary techniques can produce relatively large quantities of maceral concentrates over a wide density range. These quantities are sufficient to use infrared and NMR spectroscopic techniques for analysis. Solid-state I3C NMR spectroscopy reveals a wide variation of carbon aromaticity with density, ranging from 0.37 for the lightest fraction (1.20 fl) to 0.92 for the 1.44-1.45 g cm-3 fraction. These values are slightly lower than the few reported values of comparable density for carboniferous coals. Infrared spectroscopy reveals a rapidly changing C-H structure across the density range. The lightest fractions me highly aliphatic with a high proportion of CH2groups. With increasing density the fraction of signal from aromatic C-H increases markedly above a density of 1.41 g cmS, as does the signal due to CH3. These changes, seen within one coal, are broadly similar to changes seen in going from low- to high-rank coals. The change in chemical structure of coal with density clearly demonstrates the need for representative sampling when techniques such as FTIR are used which may require as little as 1 mg of sample for analysis.
-
Acknowledgment. We acknowledge the support of the
U.S.Office of Chemical Sciences, Department of Energy, under grant No DE-FG02-86ER13537. We also acknowledge Dr. M. Smyth for assistance with bore core selection. We thank Ms E. Gawronski and Mr B. Waugh for the petrographic analyses and demineralization of the coal, respectively.
Reaction Pathways during Coprocessing. Reaction of Illinois No. 6 and Wyodak Coals with Lloydminster and Hondo Residua under Mild Conditions Kadim Ceylanf and Leon M. Stock* Department of Chemistry, The University of Chicago, Chicago, Illinois 60637 Received October 4, 1990. Revised Manuscript Received February 19, 1991
Illinois No. 6 and Wyodak coals were coprocessed with Lloydminster and Hondo residua. The reactions were carried out in argon or in dideuterium with and without a catalyst at 400 "C for 1 h. The coal conversions under these mild conditions ranged from 40 to 56%. Wyodak coal and Lloydminster residuum provided the highest coal conversion in uncatalyzed reactions. Spectroscopic evidence indicated that coal fragments appeared in the oil fraction at short reaction times but that they were much more abundant in the asphaltene fraction. An analysis of the results implies that coprocessing is essentially a stepwise process in which coals are degraded to asphaltenes and then to oils via hydrogen atom transfer and carbon-carbon bond fragmentation processes as well as deoxygenation reactions. Adduction reactions of the aliphatic residua and the aromatic coal occur simultaneously. The reactivity of the coal, in particular its capacity to initiate free-radical chain reactions and to donate hydrogen atoms, appears to play a key role in this chemistry. Introduction Over the past few years a number of proce%ses have been developed for the direct liquefaction of coal to produce 'Present address: Department of Chemistry, InBnCl University, 44069, Malatya, Turkey.
distillate fuels. Typically, these processes involve thermal degradation of the macromolecular structure of coal and are followed by hydrogenation to stabilize the degraded material and increase the hydrogen to carbon ratio of the products. The conversion of coal structure is thought to occw in stages.' In broad terms, structural elements that
0887-0624/91/2505-0482$02.50/00 1991 American Chemical Society
